(i) Field of the Invention
This invention relates to novel carboxyl-terminated polyolefins, more especially polybutadiene, to novel binders comprising polyether or polyester elastomers derived from such carboxyl-terminated polyolefins, more especially such carboxyl-terminated polybutadienes, and to novel composite propellants containing such novel polyether and polyester elastomers. The present invention also includes the solid rocket propellants formed from such binders.
(II) Description of the Prior Art
In the prior art, composite solid propellants having superior mechanical properties and good stability at high temperatures were obtained by cross-linking carboxyl-terminated polybutadiene with epoxides.
Thus, polybutadiene propellants have been provided which have, as a binder, an elastomer formed from a heretofore known carboxyl-terminated homo-, co-, or terpolymer of polybutadiene and a curing system which involves aziridines or epoxides either alone or in combination. This curing system with which carboxyl groups of the polybutadiene react to produce amide links when tris(2-methyl-1-aziridinyl)-phosphine oxide is the cross-linking agent or ester links when polyepoxides are the cross-linking agent, requires a curing time up to one week. In addition to the long period of cure which is necessary to obtain propellants of fair quality the aziridine has many drawbacks as it is toxic, has a tendency to degrade when subjected to prolonged heating, high relative humitity or hot conditions and it also tends to post cure, thus increasing the modulus of the binder and as such the propellants. It was found moreover, that, while the ester formed by the action of the epoxide group on the terminal carboxyl groups gave a binder having great stability, good mechanical properties at low temperatures, the elimination of the aziridines (which favored adhesion of the binder with particles of ammonium perchlorate) resulted in a propellant having a short life. It was then found that the addition of surface active agents to the polyester binders permitted the improvement of the mechanical properties at low temperatures of the propellants. Furthermore, while some of the drawbacks may be partially removed by the addition of the butylene imine derivative of trimesic acid, these imines are also unstable to heat and humidity, especially a butylene imine derivative of trimesic acid which decomposes at about 0.degree. C. and further the imines are hazardous materials as they produce dermatitis on contact with the skin. Finally, the oxidizer ammonium perchlorate which is incorporated into the binder to form the rocket propellant may also react exothermically with aziridines leading to fires if the ammonium perchlorate is not properly wetted with other ingredients of the binder. These prepolymers are very viscous and thus do not permit incorporation of a percentage of solids more than about 85%.
Attempts have therefore been made to substitute the tris(2-methyl-1-aziridinyl) phosphine oxide epoxide curing system in the binders for the propellants. One such attempt has been the production of hydroxyl-terminated polybutadiene, by the living polymer synthesis followed by treatment thereof with ethylene oxide, alcohols or water. These hydroxyl-terminated polybutadiene were usually cured with isocyanates in the presence of different catalysts. However, some of these hydroxyl-terminated polybutadienes were found to exhibit a reactivity with isocyanate which was too high for propellant use, having, amongst other things, a short pot like for certain applications.
Another attempt to replace the tris(2-methyl-1-aziridinyl) phosphine oxide epoxide curing system involved the use of carboxyl-terminated polybutadiene such as, for example, that supplied under the Trade Mark HC 434 by Thiokol Chemical Corporation which had a high percentage of cis-structure (i.e. about 32%). This carboxyl-terminated polybutadiene was reduced with lithium hydride into hydroxyl-terminated polybutadiene for curing with isocyanates; the gelation time with 2,4-tolylene diisocyanate was, however, about 3 hours at about 80.degree. F. which was considered to be much too short to be useful in the production of binders in propellant manufacture. This fast cure was attributed to the presence of primary terminal hydroxyl groups in the hydroxyl-terminated polybutadiene and moreover the purification process of the hydroxyl-terminated polybutadiene was difficult as it left about 0.15% residual solids which interfered in the curing reaction. The rapid cure of the hydroxyl-terminated polybutadiene with hexamethylene diisocyanate gave a gelation time of six hours at about 27.degree. C., which was twice that obtained with 2,4-tolylene diisocyanate and which was not completely satisfactory. An improved hydroxyl-terminated polybutadiene was, however, obtained from the carboxyl-terminated polybutadiene in which the terminal hydroxyl groups are secondary hydroxyl groups.
Nevertheless, it has been found that such carboxyl-terminated polybutadienes (e.g. those prepared in the presence of free radical initiators and known by the Trade Marks of "HC 434" of Thiokol Chemical Corporation, "Syntitico" of Synthetico Rubber Company of Japan and "HYCAR-CT" of B. F. Goodrich Chemical Company) contained measurable quantities of peroxides and glutaric acid. These impurities have been found to impart poor oxygen stabllity to the polymers. On the other hand, such prepolymers prepared by ionic reactions (e.g. those known by the Trade Marks of "Telagen CT" of General Tire Rubber Co. and "Butarez CTL" of Phillips Petroleum Co.) did not have good stability. In addition such prepolymers in general had a high viscosity (i.e. about 250-450 poises, at about 25-45 Ns/m.sup.2).
Other carboxyl-terminated saturated prepolymers which have been proposed are those known by the Trade Marks of "UTREZ" of United Technology Center, "ZL-635" of Thiokol Chemical Corporation, and "Telagen SCT" of Genera Tire and Rubber Company. These, however, were found to be unsuitable, since they were very viscous. Furthermore, they did not provide products which possessed good mechanical properties at low temperatures.
Some binders have been obtained from commercially available carboxyl-terminated polybutadiene (HC 434) after cross-linking with epoxides. Examples of suitable epoxides are those known by the Trade Marks of "Epon 812" (derivative of triglycidyl glycerol) and "Epon 828" of Shell Company, of "DER 732" (diglycidyl ether of polyoxypropylene glycol) of the Dow Chemical Company, of "ERL 4221" (3,4 epoxycyclohexylmethyl 3', 4'-epoxy cyclohexane carboxylate) and "ERLA 0510" (N,N-diglycidyl-p-aminophenyl glycidyl ether) or Union Carbide Co. Many of the commercially available epoxides provided polyester binders which were only usable with difficulty to obtain propellants having high proportions of solids, because of the high viscosity of the prepolymers. When tris(2-methyl-1-aziridinyl) phosphine oxide is deleted from the formulation it is usually necessary to add surface active agents in order to improve the mechanical properties at low temperatures. In addition there is an increased curing time and a decreasing of the elongation during the course of the accelerated aging of the propellant.
As noted above, it has been proposed to cross-link the hydroxyl-terminated polybutadiene with isocyanates in order to obtain polyurethanes. Isocyanates which have usually been used in the binders for solid propellants are 2,4-toluene diisocyanate and the polymeric diisocyanate (DDl) made by General Mills Chemical Inc.
Examples of such polyurethane binders which were more frequently used commercially are those having, as prepolymer, the products known by the Trade Marks of "Telagen HT" of General Tire and Rubber Company, "Butarez HTS" of Phillips Petroleum Co. and "R45M" (hydroxyl-terminated polybutadiene) of ARCO Chemical Co. Because of its low cost and its low viscosity R45M was the product most frequently used. All the prepolymers gave polyurethane binders whose stability at high temperatures was probably less than that of polyester binders.
Another form of binder is that which is disclosed in Canadian Pat. No. 891,562 issued Jan. 25, 1972, namely an hydroxy telechelic polymer which is the reaction product of a carboxyl telechelic polymer and a mono epoxy compound.
The term "telechelic polymer" as set forth in U.S. Pat. No. 3,281,335 issued Oct. 25, 1966 to C. A. Wentz and F. E. Hopper as well as an article by the same authors entitled "Process for the Production and Purification of Carboxy Telechelic Polymers" defines polymers which are produced by polymerization of vinylidene containing monomers having reactive groups at each end of the polymer molecule. The above-identified patent provides hydroxy telechelic polybutadienes desirably rich in cis-isomer, obtained by the reaction of a mono-epoxy compound and the corresponding carboxy polymers. The term "hydroxyl-terminated" as used herein means having hydroxyl groups, including primary and secondary hydroxyl groups attached adjacent the ends of the polymer molecule and preferably at the ends of the molecule. The mono-epoxy compound may be any organic compound containing a single epoxy grouping including mono-epoxy resins; particularly preferred however are epoxy compounds of the formula: ##STR1## wherein each of R and R' is hydrogen, aryl, or an alkyl, preferably a lower alkyl group. Particularly there may be mentioned 1,2-alkylene oxides such as, for example, propylene oxide and 1,2-butylene oxide which produce secondary hydroxyl groups when reacted with the carboxyl telechelic polymer.
A typical molecule of the hydroxyl-terminated polybutadiene rich in cis-isomer according to the aforesaid patent prepared by the reaction of the large molar excess of propylene oxide with the carboxyl-terminated polybutadiene rich in cis-isomer supplied under the Trade Mark HC 434 which has a molecular weight of about 3,800 is believed to be as follows: ##STR2##
The above-identified patent also provided an elastomeric binder for use in forming a castable propellant comprising (a) a hydroxyl-telechelic polymer, particularly a hydroxyl-terminated polybutadiene rich in cis-isomer, which is the reaction product of a carboxyl telechelic polymer, particularly a carboxyl-terminated polybutadiene rich cis-isomer, and a mono-epoxide particularly a 1,2-alkylene oxide such as, for example, propylene oxide and 1,2-butylene oxide and (b) a tri-isocyanante or (c) a diisocyanate such as, for example, tolylene diisocyanate together with a triol cross-linking agent, with a tri-isocyanate cross-linking inherently occurring to form the binder. The above-identified patent also included a solid polybutadiene propellant comprising such elastomeric binder, finely divided ammonium or potassium perchlorate and finely divided aluminum.
However, presently available binders derived from hydroxyl-terminated polybutadiene and diisocyanate or carboxyl-terminated polybutadiene and epoxide-tris(2-methyl-1-aziridinyl) phosphine oxide curing agents suffer the disadvantage that they have weak stability with respect to oxidizing agents.